One of the most common causes of retinal detachment is proliferative vitreoretinopathy, an intraocular, non-malignant cellular proliferation. This process results ultimately in a separation of the retina from the retinal pigment epithelium, or RPE, because of tractional forces applied directly to the inner and outer retinal surfaces. This is the major cause for failure of retinal re-attachment surgery. (Ophthalmology, Vol. 90, pgs. 121-123 (1983)).
Proliferative vitreoretinopathy is characterized by the formation of contractile cellular membranes on both sides of the retina. (Clarkson, et al., Am. J. Ophthalmol., Vol. 84, pgs. 1-17 (1977); Constable, Trans. Ophthalmol. Soc. U.K., Vol. 95, pgs. 382-386 (1975); Constable, et al., Retina Congress, Pruett, et al., eds., pgs. 245-257, Appleton-Century-Crofts, New York (1972); Daicker, et al., Graefe's Arch. Klin. Exp. Ophthalmol., Vol. 210, pgs. 109-120 (1979); Fastenberg, et al., Am. J. Ophthalmol., Vol. 93, pgs. 565-572 (1982); Glaser, et al., Ophthalmology, Vol 94, pgs. 327-332 (1987); Green, et al., Trans. Ophthalmol. Soc. U.K., Vol. 99, pgs. 63-77 (1979); Kampik, et al., Arch. Ophthalmol., Vol. 99, pgs. 1445-1454 (1981); Machemer, et al., Am. J. Ophthalmol., Vol. 85, pgs. 181-191 (1978)). While the pathobiology of proliferative vitreoretinopathy is not clear, it appears that RPE cells are key to the development of these membranes. (Green, et al., 1979; Kampik, et al., 1981; Machemer, et al., 1978; Laqua, et al., Am. J. Ophthalmol., Vol. 80, pgs. 602-618 (1975); Hiscott, et al., Br. J. Ophthalmol., Vol. 68, pgs. 708-715 (1984)). A large body of evidence supports the concept that previously quiescent RPE cells, when displaced into the vitreous cavity and exposed to the appropriate combination of cytokines, will divide and differentiate. This differentiation results in cells having myofibroblastic characteristics including adhesiveness and contractility. As these membranes form tight adhesions with the retinal surfaces, tractional forces are generated and detachment ensues.
Most evidence indicates retinal tears as the pathway through which RPE cells move in order to enter the vitreous cavity. (Hiscott, et al., 1984), and there is a clear association between the size of a retinal tear and the incidence of proliferative vitreoretinopathy. (Ryan, Am. J. Ophthalmol., Vol. 100, pgs. 188-193 (1985)). Most likely, the RPE cells remain attached to the retinal flap as the retina is displaced into the vitreous cavity or are introduced into the vitreous cavity following cryotherapy of the retina and RPE during retinal detachment repair. (Yoshizumi, et al., Retinal Diseases, Ryan, et al., eds., New York, Grune & Stratton (1984); Campochiaro, et al., Arch. Ophthalmol., Vol. 103, pgs. 434-436 (1985); Hilton, et al., Arch. Ophthalmol., Vol. 91, pgs. 445-450 (1974)).
Viable retinal pigment epithelial cells, displaced into the vitreous cavity, are exposed to a wide variety of proteins, cytokines, and chemoattractants. Extracellular matrix proteins have profound effects on cell morphology and behavior (Glaser, et al., Ophthalmology, Vol. 100, pgs. 466-470 (1993)). RPE cells, when exposed in vitro to the extracellular matrix proteins and collagensi found in the vitreous, change from their typical epithelial cell morphology to a mesenchymal or fibroblast-like morphology. (Kampik, et al., 1981; Hay, et al., Cell Biology of Extracellular Matrix, New York, Plenum Press (1982); Vidaurri-Leal, et al., Arch. Ophthalmol., Vol. 102, pgs. 1220-1223 (1984); Laqua, et al., Am. J. Ophthalmol., Vol. 80, pgs. 913-929 (1975); Machemer, et al., Am. J. Ophthalmol., Vol. 80, pgs. 1-23 (1975)).
While considerable data regarding the RPE cell's role in proliferative vitreoretinopathy has been gathered, other cells also are involved. (Campochiaro, et al., Arch. Ophthalmol., Vol. 103, pgs. 1403-1405 (1985); Van Horn, et al., Am. J. Ophthalmol., Vol. 84, pgs. 383-393 (1977); Hiscott, et al., Br. J. Ophthalmol., Vol. 68, pgs. 698-707 (1984)). Glial cells, monocytes, and macrophages are seen in immunohistopathologic preparations of membranes removed from patients with proliferative vitreoretinopathy. Recent evidence suggests that a cellular-signaling dialogue can occur between these divergent cell types whereby cells release cytokines which induce the production of cell surface receptors for a variety of growth factors on neighboring cells. (Campochiaro, et al., Arch. Ophthalmol., Vol. 102, pgs. 1830-1833 (1984); Campochiaro, et al., Arch. Ophthalmol., Vol. 103, pgs. 576-579 (1985); Yamada, Ann. Rev. Biochem., Vol. 52, pgs. 761-799 (1983); Connor, et al., Invest. Ophthalmol. Vis. Sci., Vol. 29, pg. 307 (1988)). The cells become responsive to these new factors, and thus behave differently and differentiate further. Morphologically, each of these cell types is found intimately apposed to other cell types in these membranes.
In summary, proliferative vitreoretinopathy is a process of cellular proliferation which, if left unchecked, ultimately will result in a detachment of the retina from the RPE. This process takes place within the eye, where cell division normally does not occur after completion of development. A variety of cell types is involved, including RPE cells, glial cells, monocytes, and macrophages. The pathobiology of proliferative vitreoretinopathy, while not understood completely, involves the exposure of previously quiescent cells to factors which promote abnormal differentiation and cell division. This differentiation results in adhesive cells which contract in an unregulated, disorganized fashion and produce the tractional forces which detach the retina.
The current treatment for PVR is vitreoretinal surgery. Although such treatment often is successful, recurrent vitreoretinal traction may result in redetachment. The resulting retinal detachment sometimes causes permanent impairment of visual function. Thus, pharmacologic and other forms of therapy to inhibit recurrent membrane formation are needed.
Retroviral vectors including a negative selective marker, in particular the Herpes Simplex Virus thymidine kinase (HSV-TK) gene, have been used for treating tumors. (Moolten, et al., J. Nat. Cancer Inst., Vol. 82, pgs. 297-300 (1990); Moolten, et al., Human Gene Therapy, Vol. 1, pgs. 125-134 (1990); Plautz, et al., New Biologist, Vol. 3, pgs. 709-715 (1991); Culver, et al., Science, Vol. 256, pgs. 1550-1552 (1992); Ram, et al., J. Neurosurg., Vol. 81, pgs. 256-260 (1994)). Herpes Simplex Virus thymidine kinase confers sensitivity to the guanosine analogue, ganciclovir, which inhibits DNA synthesis and eliminates proliferating cells. (Smith, et al., Antimicrob. Agents Chemother., Vol. 22, pgs. 55-61 (1982); Field, et al., Proc. Nat. Acad. Sci., Vol. 80, pgs. 4139-4143 (1983)). It has been demonstrated that experimental PVR induced in rabbits by intraocular injection of fibroblasts bearing the HSV-TK gene was inhibited by treatment with ganciclovir (Sakamoto, et al., Ophthalmology, Vol. 102, pgs. 1417-1424 (1995)).
It has been reported that HSV-TK transduced cells are toxic to nearby nontransduced cells that were resistant to ganciclovir. Such phenomenon is termed the “bystander effect.” (Culver, et al., 1992; Ram, et al., Cancer Research, Vol. 53, pgs. 83-88 (1993); Freeman, et al., Cancer Research, Vol. 53, pgs. 5247-5283 (1993)). Sakamoto, et al., (1995) also showed a significant bystander effect in their work with experimental PVR in the rabbit model.